The desirability and need to accurately measure or control combustion properties such as constituent gas concentrations and temperature resulting from the combustion of various hydrocarbons is of increasing importance in diverse fields for assuring optimum combustion conditions. The use of tunable diode laser absorption spectroscopy (“TDLAS”) for combustion monitoring and control has been described with respect to coal-fired boilers and jet engines in a number of patent applications including WO 2004/090496, published Oct. 21, 2004 and entitled “Method and Apparatus for the Monitoring and Control of Combustion”, WO 2005/103781, published Nov. 3, 2005, entitled “Optical Mode Noise Averaging Device”, WO 2009/061586, published May 14, 2009, entitled “In Situ Optical Probe and Methods” and WO 2007/087081, published Aug. 2, 2007, entitled “Method and Apparatus for Spectroscopic Measurements in the Combustion Zone of a Gas Turbine Engine,” the content of each of which is incorporated herein in its entirety for all matters that are disclosed therein. Each of the disclosed combustion monitoring apparatus and methods feature the transmission and receipt of laser light through or near a combustion zone or related gas zones. Laser spectroscopy allows the user to measure combustion properties in a measurement zone. TDLAS techniques can be implemented in situ and offer many advantages including high speed feedback suitable for dynamic process control and environmental robustness.
TDLAS is typically implemented with tunable diode lasers operating in the near-infrared and mid-infrared spectral regions. TDLAS monitoring techniques are based on a predetermined relationship between the quantity and nature of laser light received by a detector after the light has been transmitted through a region of interest and absorbed in specific spectral bands which are characteristic of the gas species resulting from combustion. The absorption spectrum received by the detector may used to determine the quantity of a gas species under analysis plus associated combustion parameters such as temperature. There are, however, technical difficulties associated with TDLAS. One of these is the need to mitigate mode and speckle noise resulting from the transmission and receipt of the laser light.
One particular area of concern is the measurement of combustion parameters in jet aircraft engines. WO 2007/087081 describes a system and method of launching and receiving a laser signal to allow the user to measure temperature downstream of the combustion zone in a gas turbine engine. In order to minimize the optical access required and thereby minimize the number of holes in the engine casing and in order to make the optical probe as robust as possible, the architecture of the probe requires that laser light emitted from the probe be reflected off a surface in the measurement zone and back to the probe. In certain applications, to minimize the risk of misalignment, the reflecting surface is treated to provide for a Lambertian reflection. Such a surface is referred to herein as a “Lambertian surface.” For example, in those instances where the probe is reflecting light off an internal engine surface, such as an inner casing, a turbine blade or turbine shaft, the internal engine surface may be covered with a relatively rough thermal barrier coating which provides the Lambertian reflection. Lambertian reflection scatters light at the time of the reflection. Although Lambertian reflection can greatly decrease the signal levels available for detection, it also reduces alignment sensitivity since the light is scattered more or less equally into a half sphere of Π steradians.
In addition to the reduced signal level, reflection from a Lambertian surface causes an additional difficulty with regard to the signal to noise ratio of the measurement. Laser speckle noise appears in the reflected/scattered signal causing time-dependent undulating waves in the wavelength spectrum received from the laser. This makes fitting an absorption spectrum to the signal very difficult and subject to error. In the case of reflection received from a Lambertian (roughened) surface, the speckle noise is an interference phenomenon substantially similar in cause and effect to mode noise created within a multimode fiber. Light reflected from the Lambertian surface travels different distances in order to reach the receiving optic. When light waves on a first path interfere with those on another path, it causes fluctuating regions of high and low intensity creating the time-dependent undulating waves in the spectrum. These waves make it difficult to distinguish absorption of the wavelengths of interest by monitored gases from losses associated with speckle noise.
Various embodiments disclosed herein are intended to overcome one or more of the problems discussed above or other mode noise problems in TDLAS detection apparatus and methods.